1. Field of the Invention
The invention relates to a clock generator, and more particularly to a clock generator utilized in a power saving mode of a mobile communication device.
2. Description of the Related Art
A clock signal utilized in mobile communication applications has to be very precise so as to provide a stable clock signal for establishing the communications. A crystal oscillator is one common clock generator.
A mobile communication device generally requires at least two clock signals with different oscillating frequencies, where one is used for establishing communications and the other one is used for keeping accurate time of the mobile communications device. Take a Code Division Multiple Access (CDMA) communication system as an example, system frequency for establishing communication is about 19.2 MHz, while frequency of a real time clock for keeping accurate time is only about 32.768 KHz, which is much lower than 19.2 MHz. Therefore, at least two crystal oscillators are generally required by a mobile communication device, wherein one is utilized to provide a high frequency oscillating signal to establish communication channels, and the other one is utilized to provide a low frequency oscillating signal as the real time clock.
However, increased power consumption is required to support operations of two crystal oscillators. Even when the mobile communication device is in a power saving mode, such as a deep sleep mode or a real time clock (RTC) mode, the crystal oscillator providing the low frequency oscillating signal must still remain operational so that after the mobile communication device wakes up and returns to a normal mode, accurate timing thereof can be recovered. Thus, the non-stop low frequency crystal oscillator keeps consuming battery power throughout its operation, which shortens the standby time of the mobile communication device.
In addition, the cost of a crystal oscillator is very expensive. Therefore, a clock device that is capable of providing accurate timing for a mobile communications device while also saving power and reducing manufacturing costs is highly required.